ATOMIC WEIGHTS of the ELEMENTS 1999 (IUPAC Technical Report)

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INORGANIC CHEMISTRY DIVISION COMMISSION ON ATOMIC WEIGHTS AND ISOTOPIC ABUNDANCES*

ATOMIC WEIGHTS OF THE ELEMENTS 1999 (IUPAC Technical Report)

Prepared for publication by T. B. COPLEN

U.S. Geological Survey, 431 National Center, Reston, Virginia 20192, USA

*Membership of the Commission for the period 1998–1999 was as follows: Chairman: L. Schultz (Germany); Secretary: R. D. Vocke, Jr. (USA); Titular Members: J. K. Böhlke (USA); M. Ebihara (Japan); R. D. Loss (Australia); G. Ramendik (Russia); P. D. P. Taylor (Belgium); Associate Members: T. Ding (China); A. N. Halliday (USA); H.-J. Kluge (Germany); D. J. Rokop (USA); M. Stiévenard (France); S. Yoneda (Japan); National Representatives: P. De Bièvre (Belgium); J. R. de Laeter (Australia); Y. Do (Korea); N. N. Greenwood (UK); M. Shima (Japan); Y. K. Xiao (China).

Republication or reproduction of this report or its storage and/or dissemination by electronic means is permitted without the need for formal IUPAC permission on condition that an acknowledgment, with full reference to the source along with use of the copyright symbol ©, the name IUPAC, and the year of publication, are prominently visible. Publication of a translation into another language is subject to the additional condition of prior approval from the relevant IUPAC National Adhering Organization.

667 668 T. B. COPLEN

Atomic weights of the elements 1999

(IUPAC Technical Report)

Abstract: The biennial review of atomic-weight, Ar(E), determinations and other cognate data have resulted in changes for the standard atomic weights of the following elements: From To nitrogen 14.006 74 ± 0.000 07 14.0067 ± 0.0002 sulfur 32.066 ± 0.006 32.065 ± 0.005 chlorine 35.4527 ± 0.0009 35.453 ± 0.002 germanium 72.61 ± 0.02 72.64 ± 0.01 xenon 131.29 ± 0.02 131.293 ± 0.006 erbium 167.26 ± 0.03 167.259 ± 0.003 uranium 238.0289 ± 0.0001 238.028 91 ± 0.000 03

Presented are updated tables of the standard atomic weights and their uncertainties estimated by combining experimental uncertainties and terrestrial vari- abilities. In addition, this report again contains an updated table of relative atomic-mass values and half-lives of selected radioisotopes. Changes in the evaluated isotopic abundance values from those published in 1997 are so minor that an updated list will not be published for the year 1999. Many elements have a different isotopic composition in some nonterrestrial materials. Some recent data on parent nuclides that might affect isotopic abundances or atomic-weight values are included in this report for the information of the interested scientific community.

INTRODUCTION The Commission on Atomic Weights and Isotopic Abundances met under the chairmanship of Prof. L. Schultz from 8–10 August 1999 during the 40th IUPAC General Assembly in Berlin, Germany. The Commission decided to publish the report “Atomic Weights of the Elements 1999” as presented here. The resulting current Table of Standard Atomic Weights is given in alphabetical order of the prin- cipal English names in Table 1 and in order of atomic number in Table 2. The atomic weights reported in Tables 1 and 2 are for atoms in their electronic and nuclear ground states. The Commission reviewed the literature over the previous two years since the last report on atomic weights [1] and evaluated the published data on atomic weights and isotopic compositions on an element-by-element basis. The atomic weight, Ar(E), of element E can be determined from a knowledge of the isotopic abundances and corresponding atomic masses of the nuclides of that element. Compilations of the abundances of the isotopes were published in 1998 [2], and the atomic-mass eval- uations of 1993 [3] have been used by the Commission. The Commission periodically reviews the his- tory of the atomic weight of each element, emphasizing the relevant published scientific evidence on which decisions have been made [4]. The Commission wishes to emphasize the need for new precise calibrated isotopic composition measurements in order to improve the accuracy of the atomic weights of a number of elements, which are still not known to a satisfactory level of accuracy. For many elements, the limited accuracy of measurements, however, is overshadowed by terrestrial variability, which is combined in the tabulated uncertainty of the atomic weights.

For all elements for which a change in the Ar(E) value or its uncertainty, U[Ar(E)] (in parentheses, following the last significant figure to which it is attributed), is recommended, the Commission by custom makes a statement on the reason for the change and includes a list of past recommended values over a period in excess of the last 100 years, which are taken from Coplen and Peiser, 1998 [5]. Values before the formation of the International Committee on Atomic Weights in 1900 come from F. W. Clarke [6]. The names and symbols for those elements with atomic numbers 110 to 118 referred to in the following tables are systematic and based on the atomic numbers of the elements recommended for provisional use by the IUPAC Commission on the Nomenclature of Inorganic Chemistry [7]. These systematic names and symbols will be replaced by a permanent name approved by IUPAC, once the priority of dis- covery is established and the name suggested by the discoverers is examined and reviewed. The name is derived directly from the atomic number of the element using the following numerical roots: 1 un 2 bi 3 tri 4 quad 5 pent 6 hex 7 sept 8 oct 9 enn 0 nil The roots are put together in the order of the digits that make up the atomic number and termi- nated by “ium” to spell out the name. The final “n” of “enn” is deleted when it occurs before “nil,” and the final “i” of “bi” and of “tri” is deleted when it occurs before “ium.”

COMMENTS ON SOME ATOMIC WEIGHTS AND ANNOTATIONS Nitrogen The Commission has changed the recommended value for the standard atomic weight of nitrogen to Ar(N) = 14.0067(2) based on an evaluation of the variation in isotopic abundance of naturally occurring nitrogen-bearing substances. The Commission decided that the uncertainty of Ar(N) should be increased so that the implied range in relation to terrestrial variability is similar to that of other elements with significant natural isotopic variation, such as hydrogen, oxygen, and carbon. However, footnote “g” remains in Tables 1 and 2 to warn users that highly unusual naturally occurring nitrogen-bearing substances can be found with an atomic weight that falls outside the implied range. The previous value, Ar(N) = 14.006 74(7), was adopted by the Commission in 1985 [4] and was based on a change in the procedures for reporting uncertainties, taking into account mass spectrometric measurements of nitrogen in air by Junk and Svec in 1958 [8]. Historical values of Ar(N) include [5]: 1882, 14.03; 1895, 14.05; 1896, 14.04; 1907, 14.01; 1919, 14.008; 1961, 14.0067(3); and 1969, 14.0067(1).

Sulfur The Commission has changed the recommended value for the standard atomic weight of sulfur to Ar(S) = 32.065(5) based on a redetermination of the atomic weight of the Cañon Diablo troilite (CDT) reference material (32.064) and a re-evaluation of the variation in sulfur isotopic abundances of naturally occurring S-bearing substances. The new standard atomic weight and its uncertainty include almost all known values in terrestrial materials; however, footnote “g” remains to alert users to the exis- tence of rare materials with atomic weights outside that range. Sulfur has a bimodal natural atomic- weight distribution so that in practice one is more likely to find materials with either lower or higher atomic-weight values than 32.065. New calibrated mass spectrometric measurements of the isotopic composition of the IAEA-S-1 silver sulfide reference material [9,10] indicate that the atomic weight of CDT is 32.064, which is in agreement with the atomic weight of meteoritic sulfur determined by McNamara and Thode [11]. The previous standard atomic weight value, Ar(S) = 32.066(6), was adopted by the Commission in 1983 based on a compilation of isotopic variations of naturally occurring materials. Historical values of Ar(S) include [5]: 1882, 32.06; 1896, 32.07; 1903, 32.06; 1909, 32.07; 1916, 32.06; 1925, 32.064; 1931, 32.06; 1947, 32.056; 1961, 32.064(3); and 1969, 32.06(1). © 2001 IUPAC, Pure and Applied Chemistry 73, 667–683 670 T. B. COPLEN

Table 1 Standard atomic weights 1999. 12 12 [Scaled to Ar( C) = 12, where C is a neutral atom in its nuclear and electronic ground state.] The atomic weights of many elements are not invariant but depend on the origin and treatment of the material. The standard values of Ar(E) and the uncertainties (in parentheses, following the last significant figure to which they are attributed) apply to elements of natural terrestrial origin. The footnotes to this table elaborate the types of variation which may occur for individual elements and which may be larger than the listed uncertainties of values of Ar(E). Names of elements with atomic number 110 to 118 are provisional. Alphabetical order in English

Atomic Atomic Name Symbol Number Weight Footnotes

Actinium* Ac 89 Aluminium (Aluminum) Al 13 26.981 538(2) Americium* Am 95 Antimony (Stibium) Sb 51 121.760(1) g Argon Ar 18 39.948(1) g r Arsenic As 33 74.921 60(2) Astatine* At 85 Barium Ba 56 137.327(7) Berkelium* Bk 97 Beryllium Be 4 9.012 182(3) Bismuth Bi 83 208.980 38(2) Bohrium* Bh 107 Boron B 5 10.811(7) g m r Bromine Br 35 79.904(1) Cadmium Cd 48 112.411(8) g Caesium (Cesium) Cs 55 132.905 45(2) Calcium Ca 20 40.078(4) g Californium* Cf 98 Carbon C 6 12.0107(8) g r Cerium Ce 58 140.116(1) g Chlorine Cl 17 35.453(2) g m r Chromium Cr 24 51.9961(6) Cobalt Co 27 58.933 200(9) Copper (Cuprum) Cu 29 63.546(3) r Curium* Cm 96 Dubnium* Db 105 Dysprosium Dy 66 162.50(3) g Einsteinium* Es 99 Erbium Er 68 167.259(3) g Europium Eu 63 151.964(1) g Fermium* Fm 100 Fluorine F 9 18.998 4032(5) Francium* Fr 87 Gadolinium Gd 64 157.25(3) g Gallium Ga 31 69.723(1) Germanium Ge 32 72.64(1) Gold (Aurum) Au 79 196.966 55(2) Hafnium Hf 72 178.49(2) Hassium* Hs 108

Table 1 (Continued) Atomic Atomic Name Symbol Number Weight Footnotes

Helium He 2 4.002 602(2) g r Holmium Ho 67 164.930 32(2) Hydrogen H 1 1.007 94(7) g m r Indium In 49 114.818(3) Iodine I 53 126.904 47(3) Iridium Ir 77 192.217(3) Iron (Ferrum) Fe 26 55.845(2) Krypton Kr 36 83.80(1) g m Lanthanum La 57 138.9055(2) g Lawrencium* Lr 103 Lead (Plumbum) Pb 82 207.2(1) g r Lithium Li 3 [6.941(2)]† gmr Lutetium Lu 71 174.967(1) g Magnesium Mg 12 24.3050(6) Manganese Mn 25 54.938 049(9) Meitnerium* Mt 109 Mendelevium* Md 101 Mercury (Hydrargyrum) Hg 80 200.59(2) Molybdenum Mo 42 95.94(1) g Neodymium Nd 60 144.24(3) g Neon Ne 10 20.1797(6) g m Neptunium* Np 93 Nickel Ni 28 58.6934(2) Niobium Nb 41 92.906 38(2) Nitrogen N 7 14.0067(2) g r Nobelium* No 102 Osmium Os 76 190.23(3) g Oxygen O 8 15.9994(3) g r Palladium Pd 46 106.42(1) g Phosphorus P 15 30.973 761(2) Platinum Pt 78 195.078(2) Plutonium* Pu 94 Polonium* Po 84 Potassium (Kalium) K 19 39.0983(1) Praseodymium Pr 59 140.907 65(2) Promethium* Pm 61 Protactinium* Pa 91 231.035 88(2) Radium* Ra 88 Radon* Rn 86 Rhenium Re 75 186.207(1) Rhodium Rh 45 102.905 50(2) Rubidium Rb 37 85.4678(3) g Ruthenium Ru 44 101.07(2) g Rutherfordium* Rf 104 Samarium Sm 62 150.36(3) g Scandium Sc 21 44.955 910(8) Seaborgium* Sg 106 Selenium Se 34 78.96(3) r

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Table 1 (Continued) Atomic Atomic Name Symbol Number Weight Footnotes

Silicon Si 14 28.0855(3) r Silver (Argentum) Ag 47 107.8682(2) g Sodium (Natrium) Na 11 22.989 770(2) Strontium Sr 38 87.62(1) g r Sulfur S 16 32.065(5) g r Tantalum Ta 73 180.9479(1) Technetium* Tc 43 Tellurium Te 52 127.60(3) g Terbium Tb 65 158.925 34(2) Thallium Tl 81 204.3833(2) Thorium* Th 90 232.0381(1) g Thulium Tm 69 168.934 21(2) Tin (Stannum) Sn 50 118.710(7) g Titanium Ti 22 47.867(1) Tungsten (Wolfram) W 74 183.84(1) Ununbium* Uub 112 Ununhexium* Uuh 116 Ununnilium* Uun 110 Ununoctium* Uuo 118 Ununquadium* Uuq 114 Unununium* Uuu 111 Uranium* U 92 238.028 91(3) g m Vanadium V 23 50.9415(1) Xenon Xe 54 131.293(6) g m Ytterbium Yb 70 173.04(3) g Yttrium Y 39 88.905 85(2) Zinc Zn 30 65.39(2) Zirconium Zr 40 91.224(2) g

*Element has no stable nuclides. One or more well-known isotopes are given in Table 3 with the appropriate relative atomic mass and half-life. However, three such elements (Th, Pa, and U) do have a characteristic terrestrial isotopic composition, and for these an atomic weight is tabulated. †Commercially available Li materials have atomic weights that range between 6.939 and 6.996; if a more accurate value is required, it must be determined for the specific material. ggeological specimens are known in which the element has an isotopic composition outside the limits for normal material. The difference between the atomic weight of the element in such specimens and that given in the table may exceed the stated uncertainty. mmodified isotopic compositions may be found in commercially available material because it has been subjected to an undisclosed or inadvertent isotopic fractionation. Substantial deviations in atomic weight of the element from that given in the table can occur. rrange in isotopic composition of normal terrestrial material prevents a more precise Ar(E) being given; the tabulated Ar(E) value should be applicable to any normal material.

Table 2 Standard atomic weights 1999. 12 12 [Scaled to Ar( C) = 12, where C is a neutral atom in its nuclear and electronic ground state] The atomic weights of many elements are not invariant but depend on the origin and treatment of the material. The standard values of Ar(E) and the uncertainties (in parentheses, following the last significant figure to which they are attributed) apply to elements of natural terrestrial origin. The footnotes to this table elaborate the types of variation which may occur for individual elements and which may be larger than the listed uncertainties of values of Ar(E). Names of elements with atomic number 110 to 118 are provisional. Order of atomic number

Atomic Atomic Number Name Symbol Weight Footnotes

1 Hydrogen H 1.007 94(7) g m r 2 Helium He 4.002 602(2) g r 3 Lithium Li [6.941(2)]† gmr 4 Beryllium Be 9.012 182(3) 5 Boron B 10.811(7) g m r 6 Carbon C 12.0107(8) g r 7 Nitrogen N 14.0067(2) g r 8 Oxygen O 15.9994(3) g r 9 Fluorine F 18.998 4032(5) 10 Neon Ne 20.1797(6) g m 11 Sodium (Natrium) Na 22.989 770(2) 12 Magnesium Mg 24.3050(6) 13 Aluminium (Aluminum) Al 26.981 538(2) 14 Silicon Si 28.0855(3) r 15 Phosphorus P 30.973 761(2) 16 Sulfur S 32.065(5) g r 17 Chlorine Cl 35.453(2) g m r 18 Argon Ar 39.948(1) g r 19 Potassium (Kalium) K 39.0983(1) 20 Calcium Ca 40.078(4) g 21 Scandium Sc 44.955 910(8) 22 Titanium Ti 47.867(1) 23 Vanadium V 50.9415(1) 24 Chromium Cr 51.9961(6) 25 Manganese Mn 54.938 049(9) 26 Iron (Ferrum) Fe 55.845(2) 27 Cobalt Co 58.933 200(9) 28 Nickel Ni 58.6934(2) 29 Copper (Cuprum) Cu 63.546(3) r 30 Zinc Zn 65.39(2) 31 Gallium Ga 69.723(1) 32 Germanium Ge 72.64(1) 33 Arsenic As 74.921 60(2) 34 Selenium Se 78.96(3) r 35 Bromine Br 79.904(1) 36 Krypton Kr 83.80(1) g m 37 Rubidium Rb 85.4678(3) g 38 Strontium Sr 87.62(1) g r

Table 2 (Continued) Atomic Atomic Number Name Symbol Weight Footnotes

39 Yttrium Y 88.905 85(2) 40 Zirconium Zr 91.224(2) g 41 Niobium Nb 92.906 38(2) 42 Molybdenum Mo 95.94(1) g 43 Technetium* Tc 44 Ruthenium Ru 101.07(2) g 45 Rhodium Rh 102.905 50(2) 46 Palladium Pd 106.42(1) g 47 Silver (Argentum) Ag 107.8682(2) g 48 Cadmium Cd 112.411(8) g 49 Indium In 114.818(3) 50 Tin (Stannum) Sn 118.710(7) g 51 Antimony (Stibium) Sb 121.760(1) g 52 Tellurium Te 127.60(3) g 53 Iodine I 126.904 47(3) 54 Xenon Xe 131.293(6) g m 55 Caesium (Cesium) Cs 132.905 45(2) 56 Barium Ba 137.327(7) 57 Lanthanum La 138.9055(2) g 58 Cerium Ce 140.116(1) g 59 Praseodymium Pr 140.907 65(2) 60 Neodymium Nd 144.24(3) g 61 Promethium* Pm 62 Samarium Sm 150.36(3) g 63 Europium Eu 151.964(1) g 64 Gadolinium Gd 157.25(3) g 65 Terbium Tb 158.925 34(2) 66 Dysprosium Dy 162.50(3) g 67 Holmium Ho 164.930 32(2) 68 Erbium Er 167.259(3) g 69 Thulium Tm 168.934 21(2) 70 Ytterbium Yb 173.04(3) g 71 Lutetium Lu 174.967(1) g 72 Hafnium Hf 178.49(2) 73 Tantalum Ta 180.9479(1) 74 Tungsten (Wolfram) W 183.84(1) 75 Rhenium Re 186.207(1) 76 Osmium Os 190.23(3) g 77 Iridium Ir 192.217(3) 78 Platinum Pt 195.078(2) 79 Gold (Aurum) Au 196.966 55(2) 80 Mercury (Hydrargyrum) Hg 200.59(2) 81 Thallium Tl 204.3833(2) 82 Lead (Plumbum) Pb 207.2(1) g r 83 Bismuth Bi 208.980 38(2) 84 Polonium* Po 85 Astatine* At 86 Radon* Rn 87 Francium* Fr

Table 2 (Continued) Atomic Atomic Number Name Symbol Weight Footnotes

88 Radium* Ra 89 Actinium* Ac 90 Thorium* Th 232.0381(1) g 91 Protactinium* Pa 231.035 88(2) 92 Uranium* U 238.028 91(3) g m 93 Neptunium* Np 94 Plutonium* Pu 95 Americium* Am 96 Curium* Cm 97 Berkelium* Bk 98 Californium* Cf 99 Einsteinium* Es 100 Fermium* Fm 101 Mendelevium* Md 102 Nobelium* No 103 Lawrencium* Lr 104 Rutherfordium* Rf 105 Dubnium* Db 106 Seaborgium* Sg 107 Bohrium* Bh 108 Hassium* Hs 109 Meitnerium* Mt 110 Ununnilium* Uun 111 Unununium* Uuu 112 Ununbium* Uub 114 Ununquadium* Uuq 116 Ununhexium* Uuh 118 Ununoctium* Uuo

*Element has no stable nuclides. One or more well-known isotopes are given in Table 3 with the appropriate relative atomic mass and half-life. However, three such elements (Th, Pa, and U) do have a characteristic terrestrial isotopic composition, and for these an atomic weight is tabulated. †Commercially available Li materials have atomic weights that range between 6.939 and 6.996; if a more accurate value is required, it must be determined for the specific material. ggeological specimens are known in which the element has an isotopic composition outside the limits for normal material. The difference between the atomic weight of the element in such specimens and that given in the Table may exceed the stated uncertainty. mmodified isotopic compositions may be found in commercially available material because it has been subjected to an undisclosed or inadvertent isotopic fractionation. Substantial deviations in atomic weight of the element from that given in the Table can occur. rrange in isotopic composition of normal terrestrial material prevents a more precise Ar(E) being given; the tabulated Ar(E) value should be applicable to any normal material.

Chlorine The Commission has changed the recommended value for the standard atomic weight of chlorine to Ar(Cl) = 35.453(2), based on an evaluation of the variation in isotopic abundances of naturally occurring chlorine-bearing substances. The Commission decided that the uncertainty of Ar(Cl) should be increased so that the implied range in relation to terrestrial variability is similar to that of other elements with significant natural isotopic variation, such as hydrogen, oxygen, and carbon. To reflect the fact that

© 2001 IUPAC, Pure and Applied Chemistry 73, 667–683 676 T. B. COPLEN the new atomic weight of Cl and its uncertainty were assigned on the basis of known variability, the footnote “r” was added to Tables 1 and 2. The footnote “g” has been added to Tables 1 and 2 to alert users that highly unusual naturally occurring chlorine-bearing substances can be found with an atomic weight that falls outside the implied range. The previous value, Ar(Cl) = 35.4527(9), was adopted by the Commission in 1985 [4] and was based on a change in the procedures for reporting uncertainties, taking into account the calibrated mass spectrometric measurements of chlorine by Shields et al. [12]. Historical values of Ar(Cl) include [5]: 1882, 35.45; 1909, 35.46; 1925, 35.457; 1961, 35.453; and 1969, 35.453(1).

Germanium The Commission has changed the recommended value for the standard atomic weight of germanium to Ar(Ge) = 72.64(1), based on calibrated mass spectrometric measurements by Chang et al. [13]. Measurements were carried out on five germanium samples of high purity (Ge metal and GeO2) from America, Europe, and Asia using positive thermal ionization mass spectrometry. The previous value of Ar(Ge) = 72.61(2) was assigned by the Commission in 1985 [4], which considered the discrepancy between atomic weights determined by mass spectrometry and chemical methods [4,14]. By recent high quality mass spectrometric measurements, the mass spectrometric data are confirmed. Historical values of Ar(Ge) include [5]: 1894, 72.3; 1897, 72.48; 1900, 72.5; 1925, 72.60; 1961, 72.59; and 1969, 72.59(3).

Xenon The Commission has changed the recommended value for the standard atomic weight of xenon to Ar(Xe) = 131.293(6), based on a new calibrated mass spectrometric measurement of Valkiers et al. [15] on a tank of purified xenon. The footnote “g” in Tables 1 and 2 arises from the presence of naturally occurring fission products found at fossil reactors at The Gabon, Africa. The previous value, Ar(Xe) = 131.29(2), was adopted by the Commission in 1985 [4] and was based on a change in the procedures for reporting uncertainties, and it took into account the mass spectrometric measurements of Nier [16]. Historical values of Ar(Xe) include [5]: 1902, 128; 1910, 130.7; 1911, 130.2; 1932, 131.3; 1955, 131.30; 1969, 131.30(1); and 1979, 131.29(3).

Erbium The Commission has changed the recommended value for the standard atomic weight of erbium to Ar(Er) = 167.259(3), based on calibrated measurements with highly enriched erbium isotopes using positive thermal ionization mass spectrometry and a multi-collector system [17]. The value for the uncertainty takes into account the lack of adequate quantification of collector calibration. Erbium isolated from a kaolinite mineral and from four different commercially available Er-bearing materials did not show any isotope variation. Nevertheless, footnote “g” in Tables 1 and 2 arises from the presence of naturally occurring fission products found at fossil reactors at The Gabon, Africa. The previous value, Ar(Er) = 167.26(3), was adopted by the Commission in 1961, based on isotopic abundance measurements of erbium by Hayden et al. [18] and Leland [19] and atomic masses of Bhanot et al. [20]. Historical values of Ar(Er) include [5]: 1882, 166.27; 1894, 166.3; 1897, 166.32; 1900, 166.0; 1909, 167.4; 1912, 167.7; 1931, 167.64; 1934, 165.20; 1935, 167.84; 1938, 167.2; 1955, 167.22; 1961, 167.26; 1969, 167.26(3).

Uranium The Commission has changed the recommended value for the standard atomic weight of uranium to Ar(U) = 238.02891(3) based on the calibrated mass spectrometric determinations by Richter et al. [21]. These measurements reflect the improved capability of current double focusing mass spectrometers with pulse counting ion-detection systems for determination of low abundance isotopes and updated gas mass spectrometry measurements of the major isotopes. All measurements were based on internal nor- malization against well characterized isotope standards. A set of six ore samples collected worldwide was utilized for the measurements. That value applies to uranium as found in normal terrestrial sources, except as discovered in one locality in Africa (The Gabon at Oklo), hence the footnote “g” in Tables 1 and 2. The previous value, Ar(U) = 238.0289(1), was adopted by the Commission in 1979 taking into account the studies on isotopic variation by Cowan and Adler [22] and Smith and Jackson [23], and mass spectrometric measurements of White et al. [24] and Greene et al. [25]. Historical values of Ar(U) include [5]: 1882, 239.03; 1894, 239.6; 1896, 239.59; 1900, 239.6; 1903, 238.5; 1916, 238.2; 1925, 238.17; 1931, 238.14; 1937, 238.07; 1961, 238.03; and 1969, 238.029(1).

RELATIVE ATOMIC-MASS VALUES AND HALF-LIVES OF SELECTED RADIONUCLIDES For elements that have no stable or long-lived nuclides, the data on radioactive half-lives and relative atomic-mass values for the nuclides of interest and importance have been evaluated, and the recommended values and uncertainties are listed in Table 3. As has been the custom in the past, the Commission publishes a table of relative atomic-mass values and half-lives of selected radionuclides. The Commission has no prime responsibility for the dissemination of such values. There is no general agreement on which of the nuclides of the radioactive elements is, or is likely to be judged, “important”. Various criteria such as “longest half-life”, “production in quantity”, “used commercially”, have been applied in the past to the Commission’s choice. The information contained in this table will enable the user to calculate the atomic weight for radioactive materials with a variety of isotopic compositions. Atomic-mass values have been taken from the 1997 Atomic Mass Table [3]. Some of these half-lives have already been documented [26–29].

NONTERRESTRIAL DATA The isotopic abundance of elements in many nonterrestrial samples within the solar system can be measured directly by analysis of meteorites and other interplanetary materials. In recent years, the increasing sophistication of analytical instrumentation and techniques have enabled the isotopic composition of submicron sized components in nonterrestrial samples to be determined. At the same time, there has also been an increase in the use of on-board spacecraft and ground-based astronomical spectrometers to measure isotopic abundances remotely. These advances have substantially increased the number of isotopic composition data for nonterrestrial materials. The extensive analysis of nonterrestrial materials has continued to show that many elements in nonterrestrial materials have different isotopic compositions from those in terrestrial samples. Although most of the reported differences in the isotopic compositions in nonterrestrial materials are small compared with corresponding differences in normal terrestrial materials, some variations are quite large. For this reason, scientists who deal with the chemical analysis of nonterrestrial samples should exercise caution when the isotopic composition or the atomic weight of nonterrestrial samples is needed.

Table 3 Relative atomic masses and half-lives of selected radionuclides. [Prepared, as in previous years, by N. E. Holden, a former Commission member; a = year; d = day; h = hour; min = minute; s = second. Names of elements with atomic number 110 to 118 are provisional.] Atomic Element Element Mass Atomic Half-life Unit Number Name Symbol No. Mass 43 Technetium Tc 97 96.9064 4.0(3) × 106 a 98 97.9072 6.6(10) × 106 a 99 98.9063 2.1(3) × 105 a 61 Promethium Pm 145 144.9127 17.7(4) a 147 146.9151 2.623(3) a 84 Polonium Po 209 208.9824 102(5) a 210 209.9829 138.4(1) d 85 Astatine At 210 209.9871 8.1(4) h 211 210.9875 7.21(1) h 86 Radon Rn 211 210.9906 14.6(2) h 220 220.0114 55.6(1) s 222 222.0176 3.823(4) d 87 Francium Fr 223 223.0197 22.0(1) min 88 Radium Ra 223 223.0185 11.43(1) d 224 224.0202 3.66(2) d 226 226.0254 1599(4) a 228 228.0311 5.75(3) a 89 Actinium Ac 227 227.0277 21.77(2) a 90 Thorium Th 230 230.0331 7.54(3) × 104 a 232 232.0381 1.40(1) × 1010 a 91 Protactinium Pa 231 231.0359 3.25(1) × 104 a 92 Uranium U 233 233.0396 1.592(2) × 105 a 234 234.0409 2.455(6) × 105 a 235 235.0439 7.04(1) × 108 a 236 236.0456 2.342(4) × 107 a 238 238.0508 4.468(3) × 109 a 93 Neptunium Np 237 237.0482 2.14(1) × 106 a 239 239.0529 2.355(6) d 94 Plutonium Pu 238 238.0496 87.7(1) a 239 239.0522 2.410(3) × 104 a 240 240.0538 6.56(1) × 103 a 241 241.0568 14.4(1) a 242 242.0587 3.75(2) × 105 a 244 244.0642 8.00(9) × 107 a 95 Americium Am 241 241.0568 432.7(6) a 243 243.0614 7.37(2) × 103 a 96 Curium Cm 243 243.0614 29.1(1) a 244 244.0627 18.1(1) a 245 245.0655 8.48(6) × 103 a 246 246.0672 4.76(4) × 103 a 247 247.0704 1.56(5) × 107 a 248 248.0723 3.48(6) × 105 a 97 Berkelium Bk 247 247.0703 1.4(3) × 103 a 249 249.0750 3.26(3) × 102 d 98 Californium Cf 249 249.0749 351(2) a 250 250.0764 13.1(1) a 251 251.0796 9.0(5) × 102 a

Table 3 (Continued)

Atomic Element Element Mass Atomic Half-life Unit Number Name Symbol No. Mass

252 252.0816 2.64(1) a 99 Einsteinium Es 252 252.0830 472(2) d 100 Fermium Fm 257 257.0951 100.5(2) d 101 Mendelevium Md 258 258.0984 51.5(3) d 260 260.1037 27.8(3) d 102 Nobelium No 259 259.1010 58(5) min 103 Lawrencium Lr 262 262.1097 3.6(3) h 104 Rutherfordium Rf 261 261.1088 1.3a min 105 Dubnium Db 262 262.1141 34(5) s 106 Seaborgium Sg 266 266.1219 ~21a s 107 Bohrium Bh 264 264.12 0.44a s 108 Hassium Hs 277 16.5a,b min 109 Meitnerium Mt 268 268.1388 0.070a,b s 110 Ununnilium Uun 281 1.6a,b min 111 Unununium Uuu 272 272.1535 1.5a,b × 10-3 s 112 Ununbium Uub 285 15.4a,b min 114 Ununquadium Uuq 289 30.4a,b s 116 Ununhexium Uuh 289 0.60a,b × 10-3 s 118 Ununoctium Uuo 293 0.12a,b × 10-3 s a The uncertainties of these elements are unsymmetric. b The value given is determined from only a few decays.

Most isotopic variations observed in nonterrestrial materials are currently explained by the following processes: (i) Mass fractionation. Mass-dependent isotopic fractionation can also be observed in many terrestrial materials, but the magnitude of mass-dependent isotopic fractionation in nonterrestrial samples is generally larger than that found in terrestrial samples. Mass-dependent isotopic fractionation observed in nonterrestrial samples is mostly due to volatilization or condensation, which most likely occurred at the early stage of the formation of the solar system. Mass-dependent isotopic fractionation may also have occurred at later stages in the evolution of the solar system by chemical processes such as the formation of organic compounds. (ii) Nuclear reaction. Nuclear reactions triggered by cosmic rays can alter the isotopic composition of not only nonterrestrial but also terrestrial materials. This effect in terrestrial samples is nor- mally negligible, mainly because of effective shielding against cosmic rays by the Earth’s mag- netosphere and atmosphere. In contrast, nonterrestrial materials are often exposed to sufficient solar and galactic cosmic ray fluxes to cause significant nuclear spallation reactions, which in turn trigger secondary low-energy neutron capture reactions at specific depths from the surface or near the surface of samples. (iii) Radioactive decay. Isotopic anomalies due to radioactive decay are not limited to nonterrestrial samples. Nevertheless, their effect can be clearly observed in nonterrestrial samples both in the degree of the alteration of isotopic abundances and in the variety of radioactive nuclides. Enrichment in decay products is the most common effect confirmed in the nonterrestrial samples. The effect is caused by long-lived nuclides (primary radioactive nuclides), whose half-lives are comparable to the age of our solar system (4.56 × 109 a). These nuclides are commonly used in geochronology and cosmochronology.

The next observed effect caused by radioactive decay is due to parent nuclides that have decayed. While such nuclides are no longer present in the solar system, their former presence in nonterrestrial materials can be demonstrated by excesses in decay products, which together with their parent(s) have been part of a closed isotopic system. Their measurement provides valuable information related to the time from their final nucleosynthetic contribution to the solar system to their incorporation to solar system materials (also known as the solar system formation interval). In addition to these two major effects, some minor effects can be observed in nonterrestrial samples such as those caused by double ß-decay of long-lived radionuclides and nuclear fission (spontaneous and neutron-induced). (iv) Nucleosynthesis. The most significant isotopic alteration effect observed in nonterrestrial materials (mainly meteorites) is due to nucleosynthesis. Prior to the availability of secondary ion mass spectrometry (SIMS), the majority of isotopic effects due to nucleosynthesis were known to be present in only a few component samples present in a limited number of rare meteorites. The majority of these isotopic abundance variations were best ascribed to explosive nucleosynthesis

Fig. 1 Mass spectrum of Xe extracted from the Richardton meteorite [31]. Horizontal lines indicate the comparison spectrum of terrestrial Xe normalized to 132Xe. A large excess at 129Xe is due to radioactive decay of a parent nuclide, 129I (half-life = 1.6 × 107 a).

effects. Some light elements, such as oxygen and noble gas elements, showed ubiquitous isotopic anomalies (which were also considered to be related to nucleosynthetic effects) in virtually all kinds of meteoritic materials. The extensive application of the SIMS technique now shows that a large number of extra-solar materials appear to contain significant isotope anomalies. This indi- cates that several extra-solar materials appear to have survived the formation of the solar system either within meteorites or their components. Some of these components demonstrate huge isotopic variations, which are believed to be due to a wide range of nucleosynthetic effects occurring at various stages of stellar evolution. A number of examples have been listed in the previous Pure and Applied Chemistry (PAC) publications [30]. Figure 1 shows an example of how the radioactive decay of a parent nuclide, 129I in this case, alters the isotopic composition of nonterrestrial materials. 129I is a typical parent nuclide and decays to 129Xe with a half-life of 1.6 × 107 a. The presence of 129I at the early solar system was first confirmed in 1960 by Reynolds, who observed an excess of 129Xe in noble gas extracted from the Richardton meteorite [31]. Since then, more than ten long-living radionuclides with half-lives less than the age of Earth have been proposed to alter the isotopic abundance of their daughter nuclide after their decay (Table 4). The isotopic composition of Xe tends to suffer other alteration processes, such as nuclear reactions (cosmic-ray-produced spallation reaction and nuclear fission reaction) and mass-dependent isotopic fractionation. Meteoritic Xe thus occasionally shows a highly complex isotopic composition.

Table 4 Examples of measurements of anomalous isotopic compositions in extraterrestrial materials arising from the earlier decay of radioisotopes. Parent Half-life Decay Mode Daughter Absolute Difference Refs Nuclide in 106 a Nuclide Between the Observed Isotopic Ratio and That Found in Normal Terrestrial Materials 26Al 0.72 ß+, EC 26Mg 0.095 (26Mg/24Mg) 32 36Cl 0.301 ß-36Ar 0.066 (38Ar/36Ar) 33 41Ca 0.10 EC 41K5.7 (41K/39K) 34 53Mn 3.7 EC 53Cr 0.0018 (53Cr/52Cr) 35 60Fe ~0.1 2ß-60Ni 0.005 (60Ni/58Ni) 36 107Pd 6.5 ß- 107Ag 0.04 (107Ag/109Ag) 37 129I16ß- 129Xe 0.48 (129Xe/131Xe) 31 135Cs 3 ß- 135Ba -0.0002 (135Ba/138Ba) 38 146Sm 103 α 142Nd 0.000 06 (142Nd/144Nd) 39 182Hf 9 s, r 182W -0.0003 (182W/184W) 40 244Pu 82 SF 131–136Xe 0.369 (136Xe/130Xe) 41 247Cm 15.8 α, ß- 235U <0.0074 (235U/238U) 42

The Commission does not attempt to systematically review the literature on the isotopic compositions of nonterrestrial materials in this report. Those who are interested in more comprehensive reviews, including specific data and additional references, should refer to Shima [43] and Shima and Ebihara [44]. A more detailed report of isotopic measurement published in this field during the last decade is in preparation.

OTHER PROJECTS OF THE COMMISSION At intervals of about six to eight years, the Commission’s Subcommittee for Isotopic Abundance Measurements publishes a summary of its biennial review of isotopic compositions of the elements as determined by mass spectrometry and other relevant methods. The subcommittee distributed their summary report at Berlin, titled “Isotopic compositions of the elements 1997” [2]. The very few and minor 1999 changes of the evaluated best values for some isotopic abundances do not justify the publication of an updated table, which, however, is always currently maintained by and available on request from the Commission. The rules that the Commission employs in assigning atomic-weight values are found in the Commission’s Technical Booklet. J. de Laeter incorporated decisions made at the previous General Assembly at Geneva to produce the 5th edition of the Commission’s Technical Booklet, which he distributed. A new addition to this booklet is a paper on the reliability of the Avogadro constant, the fac- tor that relates measurements expressed in two SI base units (mass and amount of substance) by De Bièvre and Peiser [45]. The Subcommittee for Natural Isotopic Fractionation presented a report that was discussed and modified during the Subcommittee’s meeting in Berlin, Germany, prior to the IUPAC General Assembly in Berlin. The Subcommittee will submit a report to PAC, consisting primarily of plots (where possible) that show the variation in natural isotopic abundance and atomic weight for the elements H, Li, B, C, N, O, Si, S, Cl, Fe, Cu, Se, Pd, and Te. A companion report that is expected to be longer will include numerous references and will be published as a U.S. Geological Survey Open-File Report. J. de Laeter reported on plans and progress of the Commission-approved update to the 1984 Element By Element Review [4]. Publication of this important document is planned before the next IUPAC General Assembly in 2001 in Brisbane. Recognizing that many of the terms the Commission uses are missing from the second edition of IUPAC’s book on nomenclature (the “Gold Book” [46]), the Commission asked G. Ramendik to pre- pare a list of such terms and obtain review for submission to IUPAC nomenclature commissions and inclusion in the 3rd edition of the Gold Book.

REFERENCES 1. R. D. Vocke, Jr. Pure Appl. Chem. 71, 1593–1607 (1999). 2. K. J. R. Rosman and P. D. P. Taylor. Pure Appl. Chem. 70, 217–235 (1998). 3. G. Audi and A. H. Wapstra. Nucl. Phys. A565, 1–65 (1993). 4. H. S. Peiser, N. E. Holden, P. De Bièvre, I. L. Barnes, R. Hagemann, J. R. de Laeter, T. J. Murphy, E. Roth, M. Shima, H. G. Thode. Pure Appl. Chem. 56, 695–768 (1984). 5. T. B. Coplen and H. S. Peiser. Pure Appl. Chem. 70, 237–257 (1998). 6. F. W. Clarke. J. Am. Chem. Soc. 16, 179–193 (1894); 17, 201–212 (1895); 18, 197–214 (1896); 19, 359–369 (1897); 20, 163–173 (1898); 21, 200–214 (1899); 22, 70–80 (1900). 7. IUPAC Commission on the Nomenclature of Inorganic Chemistry, Nomenclature of Inorganic Chemistry, Recommendations 1990, G. J. Leigh (Ed.), Blackwell Scientific Publications, Oxford, (1990). 8. G. Junk and H. J. Svec. Geochim. Cosmochim. Acta 14, 234–243 (1958). 9. B. W. Robinson. In Reference and intercomparison materials for stable isotopes of light elements, IAEA-TECDOC-825, International Atomic Energy Agency, Vienna, 13–29 (1995). 10. T. Ding, R. Bai, Y. Li, D. Wan, X. Zou, Q. Zhang. Science in China (Series D) 42, 45–51 (1999). 11. J. McNamara and H. G. Thode. Phys. Rev. 78, 307–308 (1950). 12. W. R. Shields, T. J. Murphy, E. L. Garner, V. H. Dibeler. J. Am. Chem. Soc. 84, 1519–1522 (1961).

13. T.-L. Chang, W.-J. Li, G.-S. Qiao, Q.-Y. Qian, Z.-Y. Chu. Int. J. Mass Spectrom. Ion Proc. 189, 205–211 (1999). 14. J. R. De Laeter, P. De Bièvre, H. S. Peiser. Mass Spectrom. Rev. 11, 193–245 (1992). 15. S. Valkiers, Y. Aregbe, P. D. P. Taylor, P. De Bièvre. Int. J. Mass Spectrom. Ion Proc. 173, 55–63 (1998). 16. A. O. Nier. Phys. Rev. 79, 450–454 (1950). 17. T.-L. Chang, M.-T. Zhao, W.-J. Li, Q.-Y Qian. Int. J. Mass Spectrom. Ion Proc. 177, 131–136 (1998). 18. R. J. Hayden, D. C. Hess, Jr., M. G. Ingram. Phys. Rev. 77, 299 (1950). 19. W. T. Leland. Phys. Rev. 77, 634–640 (1950). 20. V. B. Bhanot, W. B. Johnson, A. O. Nier. Phys. Rev. 120, 235–251 (1960). 21. S. Richter, A. Alonso, P. D. P. Taylor. Int. J. Mass Spectrom. Ion Proc. 193, 9–14 (1999). 22. G. A. Cowan and H. H. Adler. Geochim. Cosmochim. Acta 40, 1487–1490 (1976). 23. R. F. Smith and J. M. Jackson. U.S.A.E.C. Rep KY-581 (1969). 24. F. A. White, T. L. Collins, F. M. Rourke. Phys. Rev. 101, 1786–1791 (1956). 25. R. E. Greene, C. A. Kienberger, A. B. Merservey. Report K-1201 (1955) as reported by Gladys H. Fuller, Appendix 2, “Relative Isotopic Abundances,” in 1959 Nuclear Data Tables, U.S. National Academy of Sciences, National Research Council, Washington, DC. 26. N. E. Holden. Pure Appl. Chem. 61, 1483–1504 (1989). 27. N. E. Holden. Pure Appl. Chem. 62, 941–958 (1990). 28. N. E. Holden. “Table of the Isotopes,” in CRC Handbook of Chemistry and Physics, 79th ed., Sec. 11, pp. 41–149, CRC Press, Boca Raton, FL (1998), and updates. 29. N. E. Holden and D. C. Hoffman. Pure Appl. Chem. 72, 1525–1562 (2000). 30. IUPAC Commission on Atomic Weights and Isotopic Abundances. Pure Appl. Chem. 66, 2423–2444 (1994). 31. J. H. Reynolds. Phys. Rev. Lett. 4, 8–10 (1960). 32. T. Lee, D. A. Papanastassiou, G. J. Wasserburg. Astrophys. J. 211, L107–L110 (1977). 33. S. V. S. Murty, J. N. Goswami, A. Shukolyukov. Astrophys. J. 475, L65–L68 (1997). 34. G. Srinivasan, A. A. Ulyanov, J. N. Goswami. Astrophys. J. 431, L67–L70 (1994). 35. G. W. Lugmair and A. Shukolyukov. Geochim. Cosmochim. Acta 62, 2863–2886 (1998). 36. A. Shukolyukov and G. W. Lugmair. Science 259, 1138–1142 (1993). 37. W. R. Kelly and G. J. Wasserburg. Geophys. Res. Lett. 5, 1079–1082 (1978). 38. M. T. McCulloch and G. J. Wasserburg. Astrophys. J. 220, L15–L19 (1978). 39. G. W. Lugmair and K. Marti. Earth Planet. Sci. Lett. 35, 273–284 (1977). 40. C. L. Harper and S. B. Jacobsen. Lunar Planet. Sci. XXV, 509–510 (1994). 41. M. W. Rowe and P. K. Kuroda. J. Geophys. Res. 70, 709–714 (1965). 42. J. H. Chen and G. J. Wasserburg. Earth Planet. Sci. Lett. 52, 1–15 (1981). 43. M. Shima. Geochim. Cosmochim. Acta 50, 577–584 (1986). 44. M. Shima and M. Ebihara. J. Mass Spectrom. Soc. Jpn. 37, 1–31 (1989). 45. P. De Bièvre and H. S. Peiser. Metrologia 34, 49–59 (1997). 46. A. D. McNaught and A. Wilkinson. Compendium of Chemical Terminology: IUPAC Recommendations, 2nd ed., Blackwell Scientific Publications, Oxford (1997).